Silvoarable agroforestry, or alley cropping, with hybrid poplar may be viable alternative use for crop land in northern Europe idled due to surplus food production. Current trials in England suggest that alley cropping with poplar could become as profitable as annual crop monocultures if government farm support programs may be modified to include agroforestry.
Extensive poplar plantations were first established on farmland in England during the 1960's and 1970's for commercial match stick veneer production. Food crop surpluses in the 1980's lead to a resurgence of interest in alley cropping, and the first large research trials were started in 1988 with poplar and several high-value hardwood species.
In 1992, trials were established to compare poplar and crop growth under different agronomic treatments at three sites in England (Burgess et al. 1998). Unrooted cuttings of four poplar clones, including interspecific hybrids (P. deltoides x trichocarpa and P. deltoides x nigra), were planted in rows 33 ft. (10 m) apart and 21 ft. (6.4 m) within the row. In the 26 ft. (8 m) wide alleys between the trees, three different cropping treatments were tested: continuous cropping on both sides of a tree row, alternating cropping and fallow on either side of the row, and continuous fallow on both sides of the tree row. Alley crops such as wheat, barley and beans were managed under normal commercial practices. Weeds were controlled by plastic mulch along the tree row and by spraying. The poplars were pruned once, and the trials were not irrigated.
Results after 5 years showed the growth of the Beaupré clone (a P. deltoides x trichocarpa, or "TD" hybrid), averaged over all sites and treatments, to be significantly better than the other tested clones. Competition for water and nutrients reduced the height and diameter growth of the trees under the alternatively- and continuously-cropped treatments compared to the continuous fallow treatment. These results underscore the importance of locating poplar alley cropping on the best sites with adequate soil moisture available to support both trees and crops.
Compared to crop yields in open-field control plots, yields in the continuous alley cropping treatment were reduced by less than 10%. However, removing the 20% of the field that was occupied by trees, crop yield in this treatment was only 73% of control on a unit area basis.
The economics of poplar alley cropping were also studied, based on tree and crop growth data from the trial. Results indicated that cropping would only be profitable for five years under the conditions of the trial, although in general the optimum length of cropping would be heavily dependent on the expected net returns from crop production. More cost-efficient weed control, particularly in the interface between the plastic mulch and the crop, could extend the length of profitable cropping.
The potential profitability of poplar alley cropping is also influenced both by the assumed discount rate and government subsidies in England. Without any subsidies, the analysis indicated that poplar agroforestry would be more profitable than arable farming only at a discount rate of 0%. Depending on spacing and tree stocking, alley cropping systems could qualify for government subsidies in England that encourage either agriculture or forestry. If forestry grant schemes were modified to include the lower stocking rates in agroforestry, then alley cropping could be more profitable than farming at a 5% discount rate.
Based on the overall results of the 1992 and earlier trials, researchers in England have made some general recommendations for alley cropping with poplar (Beaton et al. 1999). Tree row spacing should be designed to accommodate the widest machinery used in normal farming operations; in England this is an 59 ft. (18 m) spray boom. In such case, tree rows would be spaced 66 ft. (20 m) apart, leaving 59 ft. (18 m) wide alleys between them. Optimum in-row spacing depends on the anticipated rotation length (30 years for hybrid poplar timber) and size at harvest. To reduce the effects of shading, the tree rows are best oriented on a north-south axis. Combine-harvested crops such as cereal grains, legumes and oil seeds are easily adaptable to alley cropping, but crops requiring large harvesting equipment, e.g. corn, may not be.
Trees in an alley cropping system will require intensive management, including weed control and pruning. A combination of mulching, cultivation and herbicides are recommended for weed control, the greatest challenge being cost-effective control in the area between the plastic mulch and the crop. Pruning of poplars should start three years after planting and continue to a height of 26 ft. (8 m) by the tenth year. Timely pruning is important not only to increase the volume of clearwood timber, and thus the return from the tree crop, but also to reduce shading of crops grown in the alleys and thus to extend the cropping period.
Once shading makes the production of annual crops in the alleys unprofitable, the agroforestry system could be converted to silvopastoral management through the sowing of more shade- tolerant forage species. Since the trees are already mature, the problems of livestock damage to trees normally associated with young silvopastoral systems are eliminated.
Beaton, A., L.D. Incoll and P.J. Burgess, 1999, Silvoarable agroforestry. Scottish Forestry 53(1):28-32.
Burgess, P.J., L.D. Incoll, A. Beaton, D.T. Corry, I. Seymour, J. Taylor and R.J. Evans, 1998, Final Project Report for MAFF-funded project (OC9522): Silvoarable Trials with Poplar. Cranfield University, Silsoe, U.K., 22 pp.